Temporal lobe epilepsy is a serious neurological disorder in which spontaneous seizures are accompanied by neuroanatomical and physiological abnormalities in the hippocampus. No preventative or cure is available to circumvent the disorder, and a subset of patients are resistant to pharmacological control of the seizures. Thus, novel treatment options are necessary but require further understanding of the induction and progression of the disorder. Recent studies have shown that dentate granule cells, one of only two neuron populations continually generated throughout the lifespan, are vulnerable to seizure related abnormalities. A transgenic mouse line has been created based on these finding, in which a Cre-lox system is used to selectively remove the PTEN gene from only post-natal generated granule cells. When PTEN removal was induced at post-natal day 14, mice developed spontaneous seizures and abnormal granule cells that are characteristic of an epileptic brain. Studies in this proposal will characterize the physiological changes that occur in the dentate gyrus after PTEN is removed and assess the contribution of abnormal PTEN knockout cells in the development of a hyperexcitable hippocampal network. For these experiments, an Archaerhodopsin/GFP mouse was bred with the selective PTEN knockout mouse to create triple transgenic mice that have a photosensitive receptor only on cells in which PTEN has been removed. These animals will be surgically implanted and monitored for seizure activity beginning at 4 weeks after gene removal. Field recordings from perforant path stimulation in hippocampal slices will test the network dynamics of mice before seizure activity is detected (one week after monitoring begins), after seizures first begin (approximately three weeks after monitoring begins), and after seizures have progressed in frequency (approximately five weeks after monitoring begins). Using a yellow laser (589nm), optogenetic silencing will be used to inhibit the PTEN knockout cells in the dentate gyrus during field recording paradigms to determine the role of the abnormal cells in generating hyperactive activity in hippocampal slices during the progressive stages of epilepsy. We hypothesize that 1) PTEN KO granule cells enhance network excitability in hippocampal dentate gyrus and 2) inhibition of PTEN KO cells will return the network to normal functioning. These studies will help us to understand the role of post-natally generated granule cells in epileptogenesis and determine if removing or suppressing abnormal granule cells can return epileptic networks to normal activity.